An electrophotographic photoreceptor may have various structures in agreement with prescribed characteristics or electrophotographic processes applied.
Widely employed among them is a system in which a photoreceptor comprises a support having provided thereon at least one photoconductive layer and, if necessary, an insulating layer on the surface thereof. The photoreceptor composed of a support and at least one photoconductive layer is subjected to ordinary electrophotographic processing for image formation including charging, imagewise exposure, development and, if necessary, transfer.
Electrophotographic photoreceptors have also been used widely as offset printing plate precursor for direct printing plate making. In particular, a direct electrophotographic lithographic printing system has recently been acquiring a greater importance as a system providing hundreds to thousands of prints of high image quality.
Binders to be used in the photoconductive layer should themselves have film-forming properties and capability of dispersing photoconductive particles therein, and, when formulated into a photoconductive layer, binders should exhibit satisfactory adhesion to a support. They are also required to bear various electrostatic characteristics and image-forming properties, such that the photoconductive layer may exhibit excellent electrostatic capacity, small dark decay and large light decay, hardly undergo fatigue before exposure, and stably maintain these characteristics against change of humidity at the time of image formation.
Binder resins which have been conventionally used include silicone resins (see JP-B-34-6670, the term "JP-B" as used herein means an "examined published Japanese patent application"), styrene-butadiene resins (see JP-B-35-1960), alkyd resins, maleic acid resins and polyamides (see Japanese JP-B-35-11219), vinyl acetate resins (see JP-B-41-2425), vinyl acetate copolymer resins (see JP-B-41-2426), acrylic resins (see JP-B-35-11216), acrylic ester copolymer resins (see JP-B-35-11219, JP-B-36-8510, and JP-B-41-13946), etc. However, electrophotographic photosensitive materials using these known resins suffer from any of disadvantages, such as poor affinity for photoconductive particles (poor dispersion of a photoconductive coating composition); low charging properties of the photoconductive layer; poor quality of a reproduced image, particularly dot reproducibility or resolving power; susceptibility of reproduced image quality to influences from the environment at the time of electrophotographic image formation, such as a high temperature and high humidity condition or a low temperature and low humidity condition; and insufficient film strength or adhesion of the photoconductive layer, which causes, when used as an offset master plate, release of the photoconductive layer from the support during offset printing, failing to obtain a large number of prints.
In order to improve electrostatic characteristics of a photoconductive layer, various proposals have hitherto been made. For example, it has been proposed to incorporate into a photoconductive layer a compound containing an aromatic ring or furan ring containing a carboxyl group or nitro group either alone or in combination with a dicarboxylic acid anhydride as disclosed in JP-B-42-6878 and JP-B-45-3073. However, the thus improved photosensitive materials are still insufficient with regard to electrostatic characteristics, particularly in light decay characteristics. The insufficient sensitivity of these photosensitive materials has been compensated by incorporating a large quantity of a sensitizing dye into the photoconductive layer. However, photosensitive materials containing a large quantity of a sensitizing dye suffer considerable deterioration of whiteness, which means reduced quality as a recording medium, sometimes causing deterioration of dark decay characteristics, resulting in the failure to obtain a satisfactory reproduced image.
On the other hand, JP-A-60-10254 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") suggests to control an average molecular weight of a resin to be used as a binder of the photoconductive layer. According to this suggestion, a combined use of an acrylic resin having an acid value of from 4 to 50 whose average molecular weight is distributed within two ranges, i.e., a range of from 1.times.10.sup.3 to 1.times.10.sup.4 and a range of from 1.times.10.sup.4 and 2.times.10.sup.5, would improve electrostatic characteristics, particularly reproducibility as a PPC photoreceptor on repeated use, moisture resistance and the like.
In the field of lithographic printing plate precursors, extensive studies have been conducted to provide binder resins for a photoconductive layer having electrostatic characteristics compatible with printing characteristics. Examples of binder resins so far reported to be effective for oil-desensitization of a photoconductive layer include a resin having a molecular weight of from 1.8.times.10.sup.4 to 10.times.10.sup.4 and a glass transition point of from 10.degree. to 80.degree. C. obtained by copolymerizing a (meth)acrylate monomer and a copolymerizable monomer in the presence of fumaric acid in combination with a copolymer of a (meth)acrylate monomer and a copolymerizable monomer other than fumaric acid as disclosed in JP-B-50-31011; a terpolymer containing a (meth)acrylic ester unit having a substituent having a carboxyl group at least 7 atoms distant from the ester linkage as disclosed in JP-A-53-54027; a tetra- or pentapolymer containing an acrylic acid unit and a hydroxyethyl (meth)acrylate unit as disclosed in JP-A-54-20735 and JP-A-57-202544; a terpolymer containing a (meth)acrylic ester unit having an alkyl group having from 6 to 12 carbon atoms as a substituent and a vinyl monomer containing a carboxyl group as disclosed in JP-A-58-68046; and the like.
Nevertheless, actual evaluations of the above-described resins proposed for improving electrostatic characteristics, moisture resistance and durability revealed that none of them was satisfactory for practical use in charging properties, dark charge retention, photosensitivity, and surface smoothness of a photoconductive layer.
The binder resins proposed for use in electrophotographic lithographic printing plate precursors were also proved by evaluations to give rise to problems relating to electrostatic characteristics and background staining of prints.